There has been widely known a wireless LAN (Local Area Network) which adopts a CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) as a wireless communication system communicating between a wireless access point and a wireless terminal. The IEEE802.11ac standard adopts a downlink multiuser MIMO (DL-MU-MIMO) technology that extends a MIMO (Multi-Input Multi-Output) technology. In the DL-MU-MIMO, an access point uses a technology called beam forming to be capable of data transmission to individual wireless terminals by way of beams spatially orthogonal to each other, allowing different data to be simultaneously transmitted to a plurality of wireless terminals. This makes it possible to improve a system throughput.
Additionally, in order to attain a further high-efficiency, a task group for the IEEE802.11ax successive to the IEEE802.11ac standard has investigated a technology for aiming at the high-efficiency.
One of technology candidates for that includes an uplink multiuser MIMO (UL-MU-MIMO) technology. In the UL-MU-MIMO, a plurality of wireless terminals perform data transmission at the same timing to an access point by way of beams spatially orthogonal to each other, giving high-efficiency of the uplink transmission.
In order to sufficiently obtain benefits of spatial multiplexing by the multiuser MIMO, a user multiplexing number is preferably maintained above a certain value even in retransmitting the data. For example, assume that in a new transmission, spatial multiplexing transmission is performed by four wireless terminals, where a CRC (Cyclic Redundancy Check) error occurs in data of any only of the terminals. In this case, if the relevant data only is retransmitted, as a result, user multiplex is not performed and the system throughput decreases.
It has been known, as a method of resolving that, to newly multiplex new data in addition to retransmission data to improve a usage efficiency when the access point performs retransmission in DL-MU-MIMO transmission. This allows the user multiplexing number above a certain value to be maintained even in the retransmission. Therefore, even if the retransmission occurs, the system throughput can be improved owing to the spatial multiplexing.
However, this method is specialized for the DL-MU-MIMO without taking account of the UL-MU-MIMO. In other words, in a case of the DL-MU-MIMO transmission, the access point can perform the transmission in combination of the retransmission data and the new data based on a determination by the access point on the basis of an acknowledgement response result sent as a reply from each wireless terminal. On the other hand, in a case of the UL-MU-MIMO transmission, individual wireless terminals are multiuser MIMO transmitting devices, which means that plural transmission terminals exist. For this reason, a wireless terminal wanting to transmit the new data cannot determine whether to simultaneously multiplex the relevant new data to transmit at the same time as a wireless terminal transmitting the retransmission data. Further, in the UL-MU-MIMO, there is no scheme for multiplexing and transmitting the retransmission data and the new data by a plurality of wireless terminals. Therefore, if the data retransmission occurs, the high-efficiency UL-MU-MIMO transmission maintaining the user multiplexing number above a certain value cannot be achieved. As a user multiplexing transmission scheme, there has been known, besides the MU-MIMO, an orthogonal frequency division multiple access, in which the similar problem may also occur concerning simultaneous transmission of the retransmission data and the new data from a plurality of wireless terminals by way of an uplink OFDMA (UL-OFDMA). Note that in the UL-OFDMA, a resource unit containing one or more subcarriers is used as a minimum unit of communication resource to simultaneously receive from a plurality of wireless terminals.